Joint construction of cobalt-based alloy

ABSTRACT

In the joint construction of cobalt-based alloy, a cobalt-based alloy layer  1 , in which granular or massive eutectic carbide  2  disperses, is joined to a metal of a base metal  37  via an insert metal layer  36 . For the joint construction of cobalt-based alloy, liquid phase diffusion bonding is performed at a temperature of 1100° C. for a retention time of 1 hour with an insert metal with a thickness of about 40 μm being interposed between the base metal, which is S45C carbon steel, and a cobalt-based alloy material which has granular or massive eutectic carbide with a grain size not larger than 30 μm in a matrix of cast structure and contains 1.03 wt % C, 29.73 wt % Cr, 3.86 wt % W, 2.59 wt % Ni, 2.67 wt % Fe, 0.59 wt % Si, and 0.07 wt % Mo, the balance substantially being Co. The cobalt-based alloy layer  1  after bonding contains granular or massive eutectic carbide.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a joint construction ofcobalt-based alloy. More particularly, it relates to a jointconstruction of cobalt-based alloy, which is well suited when it isapplied to a valve and a rotating apparatus having a mechanical seal.

[0003] 2. Description of the Related Art

[0004] As a technology for joining metals of the same kind or metals ofdifferent kinds, there are generally known a method in which a brazingfiller metal is inserted between an anticorrosion and wear resistingalloy and a base metal to which the alloy is to be joined and only thebrazing filler metal is melted and solidified, by which the alloy isjoined to the base metal (brazing), a method in which a molten metal issupplied to a joint portion and a base metal to be joined is alsomelted, by which a layer in which the supplied metal and the base metalare both melted in the joint portion is formed (welding), and a methodin which a very thin surface layer of a base metal is melted and amolten metal is deposited on the surface layer, by which the depositedmetal is joined to the base metal (build-up).

[0005] Also, “Section 3.4.3 Diffusion Welding” in JSME MechanicalEngineers' Handbook, 1987 ed., B2-pp.63-64 describes diffusion weldingin which smooth surfaces of two metallic materials to be joined arebrought into contact with each other, and the two metallic materials arejoined to each other by a creep phenomenon (caused by the application ofa high load) at the contact portion and sintering caused by hightemperature in the state in which the metallic materials are maintainedat a high temperature. Also, the aforementioned JSME MechanicalEngineers' Handbook describes, as one kind of diffusion welding, ajoining method for joining different metals together that does notproduce a joint interface, wherein an insert metal containing Ni isinserted between two metallic materials to be joined, and the metallicmaterials are joined to each other by applying a high load while themetallic materials are maintained at a high temperature. The insertmetal is entirely diffused into the two metallic materials to be joined.

[0006] Japanese Patent Laid-Open No. 2000-273573 describes a method inwhich a Co-based, Ni-based, or Fe-based anticorrosion and wear resistingalloy having globular or granular eutectic carbide is joined to a basemetal by brazing, welding, or diffusion welding.

[0007] The method of joining an anticorrosion and wear resisting alloyto a base metal, which is described in Japanese Patent Laid-Open No.2000-273573, has problems described below.

[0008] In the method in which an anticorrosion and wear resisting alloyis joined to a base metal by melting a brazing filler metal, the metaljoining force is weak, so that the joint portion comes off when thetemperature increases again to the brazing temperature. According toother methods (welding or diffusion welding), since the metals to bejoined are made in a molten state, the globular or granular eutecticcarbide of the anticorrosion and wear resisting alloy changes to linearor net-form eutectic carbide. As a result, the characteristics obtainedby the globular or granular eutectic carbide are lost. In diffusionwelding, since a high load is applied to the metals to be joined at ahigh temperature at which a creep phenomenon takes place, a highresidual stress is created, or a crack develops. Therefore, diffusionwelding is unsuitable as a method in which a Co-based anticorrosion andwear resisting alloy having globular or granular eutectic carbide isjoined to a base metal while the form thereof is maintained.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a jointconstruction of cobalt-based alloy, which can exhibit characteristics ofcorrosion resistance and wear resistance of a cobalt-based alloy evenafter joining.

[0010] To achieve the above object, the present invention provides ajoint construction of cobalt-based alloy in which a cobalt-based alloyportion is diffusion bonded to a base metal portion by interposing aninsert metal between the cobalt-based alloy portion having granular ormassive eutectic carbide dispersed in a matrix of metal microstructureand the base metal portion, wherein a layer of the insert metal isformed over the base metal portion, and the cobalt-based alloy portionis located over the insert metal layer.

[0011] Since the insert metal layer is formed between the cobalt-basedalloy portion and the base metal, at the time of diffusion bonding ofthe cobalt-based alloy portion and the base metal, the cobalt-basedalloy portion is less affected adversely by heat, so that granular ormassive eutectic carbide exists in the cobalt-based alloy portion afterbonding. Therefore, the cobalt-based alloy portion in the jointconstruction of cobalt-based alloy has excellent corrosion resistanceand wear resistance.

[0012] A valve that is an application example to which theabove-described joint construction of cobalt-based alloy is applied ischaracterized in that a valve seat provided in a valve casing has acobalt-based alloy portion in which granular or massive eutectic carbidedisperses in a matrix of metal microstructure and which is brought intocontact with a valve element and a body portion installed to the valvecasing; the cobalt-based alloy portion is diffusion bonded to the bodyportion by interposing an insert metal between the cobalt-based alloyportion and the body portion; and a layer of the insert metal is formedover the body portion, and the cobalt-based alloy portion is locatedover the insert metal layer. Since the insert metal layer is formed onthe valve seat, and further the cobalt-based alloy portion exists overthe insert metal layer, granular or massive eutectic carbide exists, asdescribed above, in the cobalt-based alloy portion having been diffusionbonded, and net-form eutectic carbide does not exist therein. For thisreason, the corrosion resistance and wear resistance of valve seat areimproved, and thus the surface on which the valve seat and the valveelement are in contact with each other is less liable to be subjected tocorrosion damage due to dissolved oxygen in a fluid. Therefore, thefrequency of valve maintenance is decreased, and the life of the valveis prolonged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a longitudinal sectional view showing one preferredembodiment of a joint construction of cobalt-based alloy in accordancewith the present invention;

[0014]FIG. 2 is an explanatory view showing materials constituting thejoint construction of the embodiment shown in FIG. 1;

[0015]FIG. 3 is a microphotograph of a joint portion in the jointconstruction shown in FIG. 1;

[0016]FIG. 4 is a SEM photograph showing the distribution of cobalt,which is a principal element of a cobalt-based alloy layer, in a jointportion in the joint construction shown in FIG. 1;

[0017]FIG. 5 is a SEM photograph showing the distribution of nickel,which is a principal element of an insert metal layer, in a jointportion in the joint construction shown in FIG. 1;

[0018]FIG. 6 is a SEM photograph showing the distribution of iron, whichis a principal element of a base metal, in a joint portion in the jointconstruction shown in FIG. 1;

[0019]FIG. 7 is a longitudinal sectional view of a sluice valve inaccordance with one embodiment of the present invention, to which ajoint construction of cobalt-based alloy is applied;

[0020]FIG. 8 is a longitudinal sectional view of a valve element shownin FIG. 7;

[0021]FIG. 9 is a longitudinal sectional view of a portion near a valveseat provided in a valve casing shown in FIG. 7;

[0022]FIG. 10 is a longitudinal sectional view of a check valve inaccordance with one embodiment of the present invention, to which ajoint construction of cobalt-based alloy is applied;

[0023]FIG. 11 is a longitudinal sectional view of a relief valve inaccordance with one embodiment of the present invention, to which ajoint construction of cobalt-based alloy is applied;

[0024]FIG. 12 is a longitudinal sectional view of a globe valve inaccordance with one embodiment of the present invention, to which ajoint construction of cobalt-based alloy is applied; and

[0025]FIG. 13 is a schematic view of a boiling water nuclear power plantusing the valves shown in FIGS. 7 and 10 to 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] A preferred embodiment of a joint construction of cobalt-basedalloy in accordance with the present invention will be described withreference to FIG. 1. In the joint construction of this embodiment, acobalt-based alloy layer 1 in which granular or massive eutectic carbide2 disperses is joined to a base metal 37 via an insert metal layer 36.

[0027] This joint construction of a cobalt-based alloy is obtained asdescribed below. The base metal 37 is S45C carbon steel. As shown inFIG. 2, a cobalt-based alloy material 1A is joined to the base metal 37.The cobalt-based alloy material 1A, having granular or massive eutecticcarbide with a grain size not larger than 30 μm in a matrix of castingstructure, contains 1.03 wt % C, 29.73 wt % Cr, 3.86 wt % W, 2.59 wt %Ni, 2.67 wt % Fe, 0.59 wt % Si, and 0.07 wt % Mo, the balancesubstantially being Co. When this cobalt-based alloy material 1A isjoined to the base metal 37, an insert metal 36A is used. The insertmetal 36A is a nickel-based alloy containing 4.5 wt % Si and 3.2 wt % B,the balance being Ni.

[0028] Also, the cobalt-based alloy material 1A is manufactured asdescribed below. A cobalt-based alloy which has net-form eutecticcarbide in a matrix of casting structure and contains 1.03 wt % C, 29.73wt % Cr, 3.86 wt % W, 2.59 wt % Ni, 2.67 wt % Fe, 0.59 wt % Si, and 0.07wt % Mo, the balance substantially being Co, is hot rolled at atemperature of 1050 to 1100° C., and thereby the eutectic carbide ismade in a granular or massive form with a grain size not larger than 30μm. Thus, the cobalt-based alloy material 1A is obtained.

[0029] The insert metal 36A with a thickness of about 40 μm is heldbetween the base metal 37 and the cobalt-based alloy material 1A.Specifically, in the order shown in FIG. 2, the insert metal 36A isplaced over the base metal 37, and the cobalt-based alloy material 1A isplaced over the insert metal 36A. The cobalt-based alloy material 1A ispressed on the base metal 37 by its own weight. The solidus temperatureof insert metal is about 980° C., and the liquidus temperature thereofis about 1040° C. The cobalt-based alloy material 1A is joined to thebase metal 37 by liquid phase diffusion bonding. The liquid phasediffusion bonding was performed under the conditions of bondingtemperature: 1100° C., retention time: 1 hour, degree of vacuum: 2×10⁻⁴torr, and pressure: 80 g/cm². The retention time means joining timetaken for the liquid phase diffusion bonding to finish, during which thebonding temperature, degree of vacuum, and pressure are maintained inthe above-described conditions. The insert metal 36A contains Si and B,which are melting point lowering elements, so that the melting pointthereof is lower than that of the materials to be joined (cobalt-basedalloy material 1A and base metal 37). However, during the time when thebonding temperature is maintained, Si and B diffuse into each of thematerials to be joined and thus the melting point of the insert metal36A rises, so that the solidification of the insert metal 36A proceedsduring the liquid phase diffusion bonding.

[0030] The joint construction shown in FIG. 1 is obtained by theabove-described liquid phase diffusion bonding. FIG. 3 is an opticalmicrophotograph of a joint construction portion of cobalt-based alloy ofthis embodiment shown in FIG. 1. As is apparent from FIG. 3, in thejoint construction portion of cobalt-based alloy of this embodiment, theinsert metal layer (joint layer) 36 exists between the base metal 37 andthe cobalt-based alloy layer 1. This cobalt-based alloy layer 1 containsgranular or massive eutectic carbide.

[0031] The distribution of principal elements in each layer in the crosssection of the joint construction of this embodiment shown in FIG. 1 isanalyzed using a scanning electron microscope (SEM). FIGS. 4, 5 and 6are SEM photographs showing the analysis results. FIG. 4 shows thedistribution of cobalt, which is a principal element of the cobalt-basedalloy layer 1, in the position of line A. The distribution of cobalt isshown by a wavelike curve. The cobalt contained in the cobalt-basedalloy material 1A is diffused into the insert metal layer (joint layer)36 by the aforementioned liquid phase diffusion bonding, but is scarcelydiffused into the base metal 37. FIG. 5 shows the distribution ofnickel, which is a principal element of the insert metal layer 36, inthe position of line A. The distribution of nickel is shown by awavelike curve. The nickel contained in the insert metal 36A is scarcelydiffused into the cobalt-based alloy layer 1 and the base metal 37 evenby the aforementioned liquid phase diffusion bonding. FIG. 6 shows thedistribution of iron, which is a principal element of the base metal 37,in the position of line A. The distribution of iron is shown by awavelike curve. The iron contained in the base metal 37 is diffused intothe insert metal layer (joint layer) 36, but is scarcely diffused intothe cobalt-based alloy layer 1. The positions of line A in FIGS. 4, 5and 6 are the same positions. The nickel contained in the insert metal36A is scarcely diffused into the cobalt-based alloy layer 1 and thebase metal 37 by the liquid phase diffusion bonding, but the silicon andboron contained in the insert metal 36A are diffused into thecobalt-based alloy layer 1 and the base metal 37. The cobalt-based alloylayer 1 and the base metal 37 contain Si and B diffused from the insertmetal 36A at the time of liquid phase diffusion bonding, and the insertmetal layer 36 contains Fe diffused from the base metal 37 and Codiffused from the cobalt-based alloy material 1A, so that thecobalt-based alloy layer 1 is firmly joined to the base metal 37 via theinsert metal layer 36. The silicon and boron diffused from the insertmetal 36A exist mainly in a portion near the insert metal layer 36 inthe cobalt-based alloy layer 1 and the base metal 37.

[0032] A test piece was prepared for the joint construction of thisembodiment obtained by the aforementioned liquid phase diffusionbonding, and a shearing test was conducted using the test piece. As aresult, it was found that the shearing strength of the jointconstruction is about 36 kg/mm². Furthermore, the sheared portion was aportion of base metal (carbon steel), so that the integrity of the jointconstruction of this embodiment was verified.

[0033] For the joint construction of a cobalt-based alloy of thisembodiment, the bonding temperature is 1100° C., being considerablylower than the melting points of the cobalt-based alloy material 1A andthe base metal 37, and the retention time is also as short as 1 hour asdescribed above, so that the insert metal layer 36 remains, and thus thecobalt-based alloy material 1A is not adversely affected thermally.Therefore, the cobalt-based alloy material layer 1 after being joinedhas granular or massive eutectic carbide with a grain size not largerthan 30 μm like the cobalt-based alloy material 1A before being joined,so that it has corrosion resistance and wear resistance equal to thoseof the cobalt-based alloy material 1A. In the case where the bondingtemperature rises to the melting point of the cobalt-based alloymaterial 1A and thus the cobalt-based alloy material 1A is melted, thegranular or massive eutectic carbide is lost, and the eutectic carbidecomes to have a continuous net form at the time of solidification. Thisis the state of the aforementioned raw material used to manufacture thecobalt-based alloy material 1A. The cobalt-based alloy in which eutecticcarbide of continuous net form exists is inferior in corrosionresistance and wear resistance.

[0034] As described above, by changing the eutectic carbide fromcontinuous net form to discontinuous granular or massive form, thecorrosion resistance can be increased remarkably. The grain size ofeutectic carbide should be not larger than 30 μm, preferably not largerthan 10 μm. By dividing the eutectic carbide finely in this manner, thecorrosion resistance in the Strauss test specified in JIS G0575 can beincreased by a factor of about 300 as compared with the case ofcontinuous net-form eutectic carbide.

[0035] In the above-described embodiment, the pressure, which is one ofthe conditions for liquid phase diffusion bonding, was set at 80 g/cm².The pressure has been reduced remarkably as compared with the pressurein diffusion welding; however, it is desirable to further reduce thepressure. Also, it is desirable that the joining time (theaforementioned retention time) be shorter than 1 hour. For this reason,using the cobalt-based alloy material 1A having the aforementionedcomposition, the insert metal 36A, and the base metal 37, thecobalt-based alloy material 1A was joined to the base metal 37 via theinsert metal 36A by liquid phase diffusion bonding. Of the conditionsfor liquid phase diffusion bonding, the bonding temperature and thedegree of vacuum were the same as the aforementioned conditions, but theretention time and the pressure were set at 30 minutes and 16 g/cm²respectively. The pressure of 16 g/cm² is almost equal to the own weightof the cobalt-based alloy material to be joined. The joint constructionof cobalt-based alloy obtained under such conditions exhibitedcharacteristics equivalent to those of the above-described embodiment.In this example, in which the retention time and the pressure were setat 30 minutes and 16 g/cm², respectively, the effects obtained by theabove-described embodiment can be achieved.

[0036] It is desirable that the cobalt-based alloy having fine eutecticcarbide with a grain size not larger than 30 μm be joined by diffusionbonding to the base metal 37 selected from carbon steel, low alloysteel, and stainless steel. It is especially desirable that it be joinedby liquid phase diffusion bonding. Also, when liquid phase diffusionbonding is performed, it is desirable that joining be performed byinserting the insert metal formed of a nickel-based alloy containing Siand B between the base metal 37 and the cobalt-based alloy havinggranular or massive eutectic carbide.

[0037] As the insert metal, a metal containing a melting point loweringelement such as boron (B), silicon (Si), or phosphorous (P) ispreferably used. By diffusing the melting point lowering element such asB, Si and P into the materials to be joined, the pressure at the time ofjoining can be decreased and thus deformation due to joining can be keptless as compared with the case of solid phase diffusion bonding.

[0038] In order to change the eutectic carbide distributing continuouslyin a net form to a discontinuous granular or massive form, it isdesirable to subject an alloy obtained by, for example, casting toplastic working such as hot forging and hot rolling, or to furthersubject it to heating treatment (annealing) in addition to theaforementioned plastic working. However, the method is not limited tothe above-described methods.

[0039] A sluice valve in accordance with an embodiment of the presentinvention, to which the above-described joint construction ofcobalt-based alloy is applied, will be described with reference to FIGS.7, 8 and 9. A sluice valve 2 of this embodiment has a valve casing 51,and a valve rod 103 is inserted in the valve casing 51. A valve element50 is installed to the valve rod 103. The sluice valve 2 is used in adissolved oxygen atmosphere. An annular valve seat 1 a is provided atboth sides of the valve element 50. A passage 104 in which a fluid flowsis formed in the valve casing 51. A pair of annular valve seats 1 b areinstalled to the valve casing 51 so as to face the passage 104. Bylowering the valve rod 103, the valve element 50 is lowered and insertedbetween the paired valve seats 1 b, so that the valve seats 1 a providedon the valve element 50 come into contact with the valve seats 1 b andthereby the sluice valve 2 is closed. That is to say, the flow of fluidin the passage 104 is stopped. By raising the valve rod 103, the valveelement 50 is also raised, so that the fluid flows in the passage 104.

[0040] As shown in FIG. 8, the valve seat 1 a has an annular valve seatbody 52, which is a body portion, and an annular cobalt-based alloyportion 53. The valve seat body 52 is installed to the valve element 50,and the cobalt-based alloy portion 53 is joined to the valve seat body52 via an insert metal layer (not shown). As shown in FIG. 9, each ofthe valve seats 1 b has an annular valve seat body 54, which is a bodyportion, and an annular cobalt-based alloy portion 55. The valve seatbody 54 is installed to the valve casing 51, and the cobalt-based alloyportion 55 is joined to the valve seat body 54 via an insert metal layer(not shown). Both of the valve seat bodies 52 and 54 are SCPH2(equivalent to S25C) castings. In the state in which the sluice valve 2is closed, the cobalt-based alloy portion 53 of the valve seat 1 a is incontact with the cobalt-based alloy portion 55 of the valve seat 1 b.The valve seat bodies 52 and 54 in this embodiment correspond to thebase metal 37 in the above-described joint construction of cobalt-basedalloy material.

[0041] The cobalt-based alloy portions 53 and 55 are manufactured asdescribed below. A cobalt-based alloy which has net-form eutecticcarbide in a matrix of casting structure and contains 1.1 wt % C, 29.7wt % Cr, and 4.5 wt % W is hot rolled at a temperature of 1050 to 1100°C., and thereby the eutectic carbide is made in a granular or massiveform with a grain size not larger than 30 μm. A cobalt-based alloy ringwith a thickness of 5 mm, which had been cut out of this cobalt-basedalloy, is used as the cobalt-based alloy portions 53 and 55. Thecobalt-based alloy portions 53 and 55 are made of a cobalt-based alloyhaving granular or massive eutectic carbide with a grain size not largerthan 30 μm.

[0042] The cobalt-based alloy portion 53 is pressed on the valve seatbody 52 with an insert metal being interposed therebetween by the ownweight of the cobalt-based alloy portion 53. Also, the cobalt-basedalloy portion 55 is pressed on the valve seat body 54 with an insertmetal being held therebetween by the own weight of the cobalt-basedalloy portion 55. Thus, the cobalt-based alloy portion 53 and the valveseat body 52, and the cobalt-based alloy portion 55 and the valve seatbody 54 are subjected to liquid phase diffusion bonding in the state inwhich the insert metal being held therebetween under the conditionsdescribed below. Each of the insert metals is formed of a nickel-basedalloy containing 4.5 wt % Si and 3.2 wt % B, the balance being Ni, andhas a thickness of 40 μm. The solidus temperature of insert metal isabout 980° C., and the liquidus temperature thereof is about 1040° C.

[0043] The aforementioned liquid phase diffusion bonding is performedunder the conditions of bonding temperature: 1100° C., retention time: 1hour, degree of vacuum: 2×10⁻⁴ torr, and pressure: 80 g/cm². Since theinsert metal contains Si and B, which are melting point loweringelements, the melting point thereof is lower than that of the materialsto be joined (the valve seat body 52 and the cobalt-based alloy portion53, and the valve seat body 54 and the cobalt-based alloy portion 55).However, during the time when the bonding temperature is maintained, Siand B contained in the insert metal diffuse into each of the materialsto be joined and thus the melting point of the insert metal rises, sothat the solidification of the insert metal proceeds during the bonding.Thus, the liquid phase diffusion bonding is completed.

[0044] When the cobalt-based alloy portion 53 is joined to the valveseat body 52 by liquid phase diffusion bonding, an insert metal layer isformed on the valve seat body 52, and a layer of the cobalt-based alloyportion 53 is formed on the insert metal layer. Also, when thecobalt-based alloy portion 55 is joined to the valve seat body 54 byliquid phase diffusion bonding, an insert metal layer is formed on thevalve seat body 54, and a layer of the cobalt-based alloy portion 55 isformed on the insert metal layer. The layer of the cobalt-based alloyportion 53 and the layer of the cobalt-based alloy portion 55 havegranular or massive eutectic carbide with a grain size not larger than30 μm. After joining, the cross section of joint interface was observed.As a result, a joint defect such as a void was not found, and a goodjoint state was exhibited. For the sluice valve of this embodiment,since the surface of valve seat is composed of finely granular ormassive eutectic carbide, the valve seat is less liable to be subjectedto corrosion damage due to dissolved oxygen in the fluid than the valveseat composed of net-form eutectic carbide obtained by building-up orother methods. Also, since the coming-off of the matrix of castingstructure is restrained, the progress of corrosion of valve seat isinhibited, so that the decrease in leakproofness is prevented. The layerof the cobalt-based alloy portion 53 and the layer of the cobalt-basedalloy portion 55 also have high wear resistance. The sluice valve 2 ofthis embodiment has a long life and moreover a decreased frequency ofmaintenance because the valve seat thereof has excellent corrosionresistance and wear resistance.

[0045] A check valve in accordance with another embodiment of thepresent invention, to which the above-described joint construction ofcobalt-based alloy is applied, will be described with reference to FIG.10. A check valve 3 of this embodiment has a valve casing 38, and avalve element 39 fitted with a valve element support 40 is disposed inthe valve casing 38. The valve element support 40 is attached rotatablyto the valve casing 38. A valve seat 1 d is installed to the valvecasing 38 in a position to face such as a passage 41 formed in the valvecasing 38. A valve seat 1 c is installed to the valve element 39 so asto be opposed to the valve seat 1 d. The check valve 3 allows a fluid toflow from the passage 41 toward a passage 42 in the valve casing 38, butwhen a flow of the fluid from the passage 42 toward the passage 41(reverse flow) takes place, the reverse flow of fluid is checked by thevalve element 39 pressed on the valve seat 1 d. At this time,specifically, the valve seat 1 c comes into contact with the valve seat1 d. Although not shown in the figure, the valve seat 1 c has theannular valve seat body 52, which is a body portion, and the annularcobalt-based alloy portion 53, like the aforementioned valve seat 1 a.The valve seat body 52 is installed to the valve element 39, and thecobalt-based alloy portion 53 is joined to the valve seat body 52 via aninsert metal layer. Although not shown in the figure, the valve seat 1 dhas the annular valve seat body 54, which is a body portion, and theannular cobalt-based alloy portion 55, like the aforementioned valveseat 1 b. The valve seat body 54 is installed to the valve casing 38,and the cobalt-based alloy portion 55 is joined to the valve seat body54 via an insert metal layer (not shown). Both of the valve seat bodies52 and 54 are SCPH2 (equivalent to S25C) castings. The valve seat bodies52 and 54 in this embodiment correspond to the base metal 37 in theabove-described joint construction of cobalt-based alloy.

[0046] The cobalt-based alloy portions 53 and 55 used for the checkvalve 3 are manufactured as described below. A cobalt-based alloy whichhad net-form eutectic carbide in a matrix of casting structure was hotforged at a temperature of 1050 to 1100° C. Thereby, a hard cobalt-basedalloy which had eutectic carbide divided into a granular or massive formwith a grain size not larger than 30 μm and contained 1.1 wt % C, 29.7wt % Cr, and 4.5 wt % W, the balance being Co, was obtained. A ring witha thickness of 5 mm, which had been cut out of this cobalt-based alloy,was used as the cobalt-based alloy portions 53 and 55 used for the checkvalve 3. The cobalt-based alloy portions 53 and 55 are made of acobalt-based alloy having granular or massive eutectic carbide with agrain size not larger than 30 μm.

[0047] The cobalt-based alloy portion 53 is pressed on the valve seatbody 52 with an insert metal being held therebetween by the own weightof the cobalt-based alloy portion 53. Also, the cobalt-based alloyportion 55 is pressed on the valve seat body 54 with an insert metalbeing held therebetween by the own weight of the cobalt-based alloyportion 55. Thus, the cobalt-based alloy portion 53 and the valve seatbody 52, and the cobalt-based alloy portion 55 and the valve seat body54 are subjected to liquid phase diffusion bonding in the state in whichthe insert metal being held therebetween under the conditions describedbelow. Each of the insert metals is formed of a nickel-based alloycontaining CR wt %, 3 wt % Fe, 4.5 wt % Si, and 3.2 wt % B, the balancebeing Ni, and has a thickness of 40 μm. The solidus temperature ofinsert metal is about 970° C., and the liquidus temperature thereof isabout 1090° C.

[0048] The aforementioned liquid phase diffusion bonding in thisembodiment is performed under the conditions of bonding temperature:1090° C., retention time: 1 hour, degree of vacuum: 2×10-4 torr, andpressure: 50 g/cm². As in the case of the sluice valve 2, Si and Bcontained in the insert metal diffuse into each of the materials to bejoined and thus the melting point of the insert metal rises, so that thesolidification of the insert metal proceeds during the bonding. When thecobalt-based alloy portion 53 is joined to the valve seat body 52 byliquid phase diffusion bonding, an insert metal layer is formed on thevalve seat body 52, and a layer of the cobalt-based alloy portion 53 isformed on the insert metal layer. Also, when the cobalt-based alloyportion 55 is joined to the valve seat body 54 by liquid phase diffusionbonding, an insert metal layer is formed on the valve seat body 54, anda layer of the cobalt-based alloy portion 55 is formed on the insertmetal layer. The layer of the cobalt-based alloy portion 53 and thelayer of the cobalt-based alloy portion 55 have granular or massiveeutectic carbide with a grain size not larger than 30 μm.

[0049] After joining, the cross section of joint interface is observed.As a result, a joint defect such as a void was not found, and a goodjoint state was exhibited. Since the eutectic carbide of thecobalt-based alloy portion located on the surface of valve seat is fine,the occurrence of corrosion of eutectic carbide caused by dissolvedoxygen is restrained by the check valve of this embodiment as well, sothat the coming-off of the matrix of casting structure is restrained.Therefore, the corrosion of valve seat is inhibited, so that thedecrease in leakproofness is prevented. Also in this embodiment, sinceas the insert metal, an alloy containing Cr having high corrosionresistance is used, the corrosion resistance in the joint portion,especially the corrosion resistance in the joint portion in anatmosphere of high-temperature and high-pressure water or water vaporcontaining much dissolved oxygen, can be maintained. Also, the layer ofthe cobalt-based alloy portion 53 and the layer of the cobalt-basedalloy portion 55 also have high wear resistance. The check valve 3 ofthis embodiment has a long life and more over a decreased frequency ofmaintenance because the valve seat thereof has excellent corrosionresistance and wear resistance.

[0050] A relief valve in accordance with still another embodiment of thepresent invention, to which the above-described joint construction ofcobalt-based alloy is applied, will be described with reference to FIG.11. A relief valve 43 of this embodiment has a valve casing 44, and avalve element 56, which is pressed by a valve rod 46, is provided in thevalve casing 44. The valve element 56 is not connected to the valve rod46, and is merely in contact therewith. A coil spring 47 is disposed inthe valve casing 44. The upper end of the coil spring 47 is in contactwith a spring support 48 installed to the valve casing 44, and the lowerend of the coil spring 47 is in contact with a spring support 49installed to the valve rod 46. Specifically, the coil spring 47 isdisposed between the spring support 48 and the spring support 49. Anannular valve seat 60 is disposed in the lower part of the valve casing44 so as to face a flow passage 62. The valve rod 46 presses the valveelement 56 by the action of the coil spring 47, so that the valveelement 56 is pressed on the valve seat 60. In this state, the flow offluid from the flow passage 62 to the flow passage 63 is checked. Whenthe fluid pressure increases to a value that overcomes the pressingforce of the coil spring 47, the coil spring 47 is compressed by thefluid pressure, so that the valve element 56 is pushed up. Therefore,the fluid flows from the flow passage 62 toward the flow passage 63. Thefluid with a high pressure in the flow passage 62 on the upstream sideof the valve element 56 is discharged to the outside.

[0051] The valve seat 60 has an annular valve seat body 59 and anannular cobalt-based alloy portion 61, and although not shown in thefigure, an insert metal layer exists between the valve seat body 59 andthe cobalt-based alloy portion 61 as in the case shown in FIG. 1. Thevalve element 56 is also provided with an annular valve seat 64, andthis valve seat 64 has an annular valve seat body 57 and an annularcobalt-based alloy portion 58. Although not shown in the figure, aninsert metal layer exists between the valve seat body 57 and thecobalt-based alloy portion 58 as in the case shown in FIG. 1. Both ofthe valve seat bodies 57 and 59 are SCPH2 castings. The composition ofeach of the cobalt-based alloy portions 58 and 61 is the same as thecomposition of the cobalt-based alloy portion 53 of the sluice valve 2,and the cobalt-based alloy portions 58 and 61 also have granular ormassive eutectic carbide with a grain size not larger than 30 μm. Thecomposition of an insert metal, which defines the composition of theinsert metal layer of the valve seats 60 and 64, is also the same as thecomposition of the insert metal used for the sluice valve 2. Thecobalt-based alloy portion 58 and the cobalt-based alloy portion 61 areopposed to each other. In the state in which the insert metal isinterposed between the valve seat body 57 and the cobalt-based alloyportion 58 and also the insert metal is interposed between the valveseat body 59 and the cobalt-based alloy portion 61, liquid phasediffusion bonding is performed under the same conditions as those forthe sluice valve 2.

[0052] The relief valve 43 having the above-described joint constructionof cobalt-based alloy can achieve the same effects as those of thesluice valve 2.

[0053] A globe valve in accordance with still another embodiment of thepresent invention, to which the above-described joint construction ofcobalt-based alloy is applied, will be described with reference to FIG.12. For the globe valve 65 of this embodiment, a valve rod 67 isdisposed in a valve casing 66, and an annular valve seat 69 is installedto a valve element 68. The valve element 68 is provided in the lower endportion of the valve rod 67. An annular valve seat 72 is provided in thevalve casing 66. When the valve element 68 separates from the valve seat72 and is positioned above, a fluid flowing into a flow passage 75 ofthe valve casing 66 flows upward through the valve seat 72, and goesinto a flow passage 76.

[0054] The valve seat 69 has an annular valve seat body 70 and anannular cobalt-based alloy portion 71, and although not shown in thefigure, an insert metal layer exists between the valve seat body 70 andthe cobalt-based alloy portion 71 as in the case shown in FIG. 1. Theother valve seat 72 has an annular valve seat body 73 and an annularcobalt-based alloy portion 74. Although not shown in the figure, aninsert metal layer exists between the valve seat body 73 and thecobalt-based alloy portion 74 as in the case shown in FIG. 1. Both ofthe valve seat bodies 69 and 73 are SCPH2 castings. The composition ofeach of the cobalt-based alloy portions 71 and 74 is the same as thecomposition of the cobalt-based alloy portion 53 of the sluice valve 2,and the cobalt-based alloy portions 71 and 74 also have granular ormassive eutectic carbide with a grain size not larger than 30 μm. Thecomposition of an insert metal, which defines the composition of theinsert metal layer of the valve seats 69 and 72, is also the same as thecomposition of the insert metal used for the sluice valve 2. Thecobalt-based alloy portion 71 and the cobalt-based alloy portion 74 areopposed to each other. In the state in which the insert metal isinterposed between the valve seat body 70 and the cobalt-based alloyportion 71 and also the insert metal is interposed between the valveseat body 73 and the cobalt-based alloy portion 74, liquid phasediffusion bonding is performed under the same conditions as those forthe sluice valve 2.

[0055] The globe valve 65 having the above-described joint constructionof cobalt-based alloy material can achieve the same effects as those ofthe sluice valve 2.

[0056] A schematic configuration of a boiling water nuclear power plantwill be described with reference to FIG. 13. A coolant is heated by heatgenerated in a reactor core within a reactor pressure vessel 14, turningto high-temperature and high-pressure steam, and is introduced into ahigh pressure turbine 18 after passing through a main steam pipe 15 of amain steam system. Although not shown in the figure, the main steam pipe15 is provided with the relief valve 43 shown in FIG. 11. The steamdischarged from the high pressure turbine 18 is introduced into a lowpressure turbine 19 after passing through a moisture separator 17. Bythe introduction of steam into the high pressure turbine 18 and the lowpressure turbine 19, these turbines are rotated to drive a generator 20.The electricity generated by the generator 20 is introduced to a powertransmission line after passing through a main transformer 21. The steamdischarged from the high pressure turbine 18 and the low pressureturbine 19 is condensed into water by a main condenser 10. This water isreturned as feed water into a reactor pressure vessel 14 by a feed watersystem 6 in which many valves (for example, the sluice valves 2 and thecheck valves 3) in which the joint construction of cobalt-based alloyshown in FIG. 1 is applied to the valve seat and valve element areprovided. Specifically, the feed water discharged from the maincondenser 10 is pressurized by a low pressure condensate pump 25, andthen is sent to a condensate filter 28 and a condensate demineralizer 29to be purified. Thereafter, the purified feed water is furtherpressurized by a high pressure condensate pump 36, and is sent to a lowpressure feed water heater 7. The feed water having been heated by thelow pressure feed water heater 7 is further pressurized by a feed waterpump 30, and is heated by a high pressure feed water heater 31. Then, itis returned into the reactor pressure vessel 14 after passing through afeed water pipe 9. The reactor pressure vessel 14 is disposed in areactor container 13. A reactor purification system 5 for purifying thecoolant in the reactor pressure vessel 14 has a heat exchanger 33 and afilter demineralizer 34. A boric acid spray system has an SLC tank 11and SLC pump 12. An offgas treatment system for purifying theradioactive gas separated by the main condenser 10 has an air ejector24, an activated carbon packed tower 23, and a flue 22. Further, theboiling water nuclear power plant includes a recirculation system 8, acondensate storage tank 27, a control rod drive system 32, and a reactorcore isolation cooling system 35. Although not shown in the figure, therecirculation system 8 is provided with the aforementioned sluice valve2, and the reactor purification system 5, the control rod drive system32, and the reactor core isolation cooling system 35 each are providedwith a plurality of the aforementioned sluice valves 2 and check valves3. The aforementioned globe valve 65 is also used in the aforementionedsystems of the boiling water nuclear power plant. The aforementionedmain steam system, reactor purification system 5, feed water system 6,recirculation system 8, control rod drive system 32, and reactor coreisolation cooling system 35 are systems which are connected to thereactor pressure vessel 14, that is, a reactor incorporating a reactorcore, and in which a coolant in the reactor flows.

[0057] The above-described boiling water nuclear power plant is providedwith a large number of sluice valves 2, check valves 3, and globe valves65, which have excellent corrosion resistance and wear resistance, asvalves in a dissolved oxygen atmosphere. Therefore, the frequency ofmaintenance of these valves is decreased, and also the elution of cobalt(Co) caused by the corrosion of valve seat can be restrained. As aresult, the generation of long-life Co isotopes caused by the inflow ofCo having been eluted from the valve seat into the reactor core can berestrained, so that the exposure of operators of the nuclear power plantcan be reduced. Therefore, the maintenance time required for the valvescan be decreased remarkably, so that regular inspection work at the timeof regular inspection of the boiling water nuclear power plant can bereduced, and not only the exposure dose can be decreased but also thework at the time of regular inspection of the nuclear power plant can bestreamlined. The aforementioned sluice valve 2, check valve 3, reliefvalve 43, and globe valve 65 can be installed in a pipe in each systemconnected to a reactor incorporating a reactor even in a pressurizedwater nuclear power plant.

[0058] According to the present invention, since granular or massiveeutectic carbide exists in the cobalt-based alloy portion afterdiffusion bonding, the cobalt-based alloy portion in the jointconstruction of cobalt-based alloy has excellent corrosion resistanceand wear resistance.

What is claimed is:
 1. A joint construction of cobalt-based alloy inwhich a cobalt-based alloy material portion is diffusion bonded to abase metal portion by interposing an insert metal between saidcobalt-based alloy portion, in which granular or massive eutecticcarbide disperses in a matrix of metal microstructure, and said basemetal portion, wherein a layer of said insert metal is formed over saidbase metal portion, and said cobalt-based alloy portion is located oversaid insert metal layer.
 2. The joint construction of cobalt-based alloyaccording to claim 1, wherein said base metal portion and saidcobalt-based alloy portion contain an element diffused from said insertmetal.
 3. The joint construction of cobalt-based alloy according toclaim 1 or 2, wherein said insert metal layer contains an elementdiffused from said base metal portion and cobalt diffused from saidcobalt-based alloy portion.
 4. The joint construction of cobalt-basedalloy material according to any one of claims 1 to 3, wherein the grainsize of said eutectic carbide is not larger than 30 μm.
 5. The jointconstruction of cobalt-based alloy material according to any one ofclaims 1 to 4, wherein said base metal portion is formed of any ofcarbon steel, low alloy steel, and stainless steel.
 6. The jointconstruction of cobalt-based alloy material according to any one ofclaims 1 to 5, wherein said cobalt-based alloy portion contains 0.6 to3% C, 2% or less Si, 25 to 32% Cr, 15% or less W, 0 to 3% Fe, 0 to 3%Ni, and 0 to 6% Mo by weight, the balance being Co and unavoidableimpurities.
 7. A valve comprising a valve casing and a valve elementdisposed in said valve casing, said valve casing having a valve seatwhich comes into contact with said valve element, wherein said valveseat has a cobalt-based alloy portion in which granular or massiveeutectic carbide disperses in a matrix of metal microstructure and whichis brought into contact with said valve element, and a body portioninstalled to said valve casing, said cobalt-based alloy portion isdiffusion bonded to said body portion by interposing an insert metalbetween said cobalt-based alloy portion and said body portion, and alayer of said insert metal is formed over said body portion, and saidcobalt-based alloy portion is located over said insert metal layer.
 8. Avalve comprising a valve casing and a valve element disposed in saidvalve casing, said valve casing and said valve element each having avalve seat which comes into contact with each other, wherein said valveseats each have a cobalt-based alloy portion in which granular ormassive eutectic carbide disperses in a matrix of metal microstructureand which is brought into contact with the other valve element, and abody portion installed to said valve casing, said cobalt-based alloyportion is diffusion bonded to said body portion by interposing aninsert metal between said cobalt-based alloy portion and said bodyportion, and a layer of said insert metal is formed over said bodyportion, and said cobalt-based alloy portion is located over said insertmetal layer.
 9. The valve according to claim 8, wherein said bodyportion and said cobalt-based alloy portion contain an element diffusedfrom said insert metal.
 10. The valve according to claim 8 or 9, whereinsaid insert metal layer contains an element diffused from said bodyportion and cobalt diffused from said cobalt-based alloy portion. 11.The valve according to any one of claims 8 to 10, wherein the grain sizeof said eutectic carbide is not larger than 30 μm.
 12. The valveaccording to any one of claims 8 to 11, wherein said body portion isformed of carbon steel, low alloy steel, or stainless steel.
 13. Thevalve according to any one of claims 8 to 12, wherein said cobalt-basedalloy material portion contains 0.6 to 3% C, 2% or less Si, 25 to 32%Cr, 15% or less W, 0 to 3% Fe, 0 to 3% Ni, and 0 to 6% Mo by weight, thebalance being Co and unavoidable impurities.
 14. A nuclear reactor plantcomprising a reactor incorporating a reactor core, a system in which acoolant in said reactor flows, and a valve provided in a pipe of saidsystem, wherein said valve has a valve casing and a valve elementdisposed in said valve casing, said valve casing has a valve seat whichcomes into contact with said valve element, said valve seat has acobalt-based alloy portion in which granular or massive eutectic carbidedisperses in a matrix of metal microstructure and which is brought intocontact with said valve element, and a body portion installed to saidvalve casing, said cobalt-based alloy portion is diffusion bonded tosaid body portion by interposing an insert metal between saidcobalt-based alloy portion and said body portion, and a layer of saidinsert metal is formed over said body portion, and said cobalt-basedalloy portion is located over said insert metal layer.
 15. A nuclearreactor plant comprising a reactor incorporating a reactor core, asystem in which a coolant in said reactor flows, and a valve provided ina pipe of said system, wherein said valve has a valve casing and a valveelement disposed in said valve casing, said valve casing and said valveelement each have a valve seat which comes into contact with each other,said valve seats each have a cobalt-based alloy portion in whichgranular or massive eutectic carbide disperses in a matrix of metalmicrostructure and which is brought into contact with the other valveelement, and a body portion installed to said valve casing, saidcobalt-based alloy portion is diffusion bonded to said body portion byinterposing an insert metal between said cobalt-based alloy portion andsaid body portion, and a layer of said insert metal is formed over saidbody portion, and said cobalt-based alloy portion is located over saidinsert metal layer.